Twelve million emergency department (ED) visits for respiratory problems occur annually in the United States. Because of the high charges for ED care and because of concerns that use of the ED may disrupt the continuity of care available from traditional primary care providers, much attention has been given to decreasing unnecessary ED use, especially among Medicaid patients and among those in managed care organizations. These concerns are especially relevant to use of the ED for respiratory problems. Most respiratory visits are for asthma, chronic obstructive pulmonary disease, and pneumonia, which might have been prevented by an earlier visit to a traditional primary care setting, and for upper respiratory tract infections, pharyngitis, acute bronchitis, and otitis media, which might have been handled more appropriately in a traditional primary care setting. Prior research has focused on characteristics of patients who use the ED for minor problems and has largely ignored the effect of the structure of primary care practices on ED utilization. Therefore, the policy impact of previous research has been limited, as many characteristics of patients are difficult to modify. In contrast, the structure of primary care practices is potentially amenable to intervention. Before implementing any change to achieve more appropriate EDS use, it will be necessary to determine what practice characteristics lead to potentially avoidable ED use. The proposed cohort study will examine the relationship between ED utilization for respiratory problems by patients enrolled in a Medicaid managed care organization and characteristics of their primary care providers. The principal outcomes, ED utilization rates for respiratory disease and a subset of those visits characterized as nonurgent, will be measured using claims data, to be provided by the managed care organization for all of its members at the individual level. Characteristics of primary care provider sites will be measured through site visits, supplemented by data available form the managed care organization. The study will focus on ED visits for respiratory problems, because these visits are frequent and because it is plausible that ED care for many of them can be redirected to traditional primary care settings. It is expected that the analysis will identify provider characteristics reflective of access that are associated with the rate of ED use. Identifying these associations will allow the design of interventions to decrease patients need for and use of EDs.